With radar apparatuses broadly adopted for ships, an antenna which rotates at a velocity of, for example, about 20 to 30 rpm, is installed on a ship body and used in detecting a target object existing around the ship body. Each radar apparatus transmits electromagnetic waves pulsed-modulated by the antenna, receives reflection waves reflected on the target object, and detects an azimuth and distance of the target object by using reception signals obtained from the reflection waves. Moreover, with such a radar apparatus, a measurement of a relative velocity between the target object and the ship body is performed by using the reflection waves from the target object received by the antenna.
With the radar apparatus installed in a movable body, such as a ship as above, a Doppler frequency caused by a movement of the movable body itself is generated, and therefore, a correction of error caused by the velocity of the ship or rocking/drifting of the ship is performed when measuring a ground velocity of the target object (an absolute velocity of the target object).
FIG. 11 is a conceptual view for describing a method of calculating a ground velocity of a target object by correcting a velocity of a ship. A relative velocity Vr of a target object 110 in an azimuth direction with respect to a ship 100 is observed by a radar apparatus 120 installed in the ship 100.
Since the observation that can be performed by the radar apparatus 120 is for the relative velocity Vr of the target object 110, in order to obtain a ground velocity (absolute velocity) Voc of the target object 110 in the azimuth direction, for example, as described in Patent Document 1 (JPH05-333144A), an absolute velocity of the ship 100 installed with the radar apparatus 120 and a velocity caused by rocking/drifting of the ship 100 (hereinafter, simply referred to as the ship velocity) need to be subtracted. The velocity of the ship 100 is given as information of a position of the ship 100 which changes with time. The position and velocity of the ship 100 are given from other measuring instrument, such as a GPS (Global Positioning System), a LOG (ship velocity distance meter), or a gyrocompass.
Since the velocity Vo of the ship 100 is obtained regardless of the measurement of the relative velocity Vr by the radar apparatus 120 as above, the correction needs to be performed in consideration of the azimuth and the like of the target object 110 detected by the radar apparatus 120. A direction (course) of a vector of the velocity Vo of the ship 100 measured by the other measuring instrument has an angle β with respect to the azimuth at which the target object 110 exists.
Therefore, the ground velocity Voc of the target object 110 is given based on the following equation: Voc=Vr−Vo×cos β.
The velocity Vo of the ship 100 does not match with a heading θh of the ship 100. For example, there is a case where a course θc of the ship 100 no longer matches with the heading θh due to periodic current. On the other hand, an azimuth αT of the antenna of the radar apparatus 120 is determined with reference to the heading θh of the ship 100. Therefore, the angle β formed between the direction of the vector of the velocity Vo of the ship 100 and the azimuth of the target object 110 needs to be calculated by using the azimuth αT of the antenna, as β=αT−(θc−θh).
Therefore, the ground velocity Voc of the target object 110 to be obtained can be rewritten as the following equation: Voc=Vr−Vo×cos(αT−(θc−θh)).